[rpmerf]

The difference you saw is not from octane, it is from ethanol. 87 octane has more energy stored than 91 or 94. The advantage of the higher octanes, is the higher detonation resistance. The difference in energy in 87 vs 94 isn't that large though. The difference between gasoline and ethanol is huge. Ethanol has about 2/3 the energy of gasoline.

Either way, I am surprised you even saw a difference given you are using a carburetor. The amount of liquid injected should be based on load and RPM, not how much energy is stored in the fuel. EFI would be able to determine rich / lean and adjust automatically. However, maybe the power increase means you can run with less throttle and RPM.